Abrasive grindstone

ABSTRACT

An abrasive grindstone for grinding a workpiece is disclosed. The abrasive grindstone includes diamond abrasive grains and a boron compound. The diamond abrasive grains and the boron compound are compounded at a predetermined volume ratio. The average grain size Y of the diamond abrasive grains is set to 0 μm&lt;Y≦50 μm. The average grain size ratio Z of the boron compound to the diamond abrasive grains is set to 0.8≦Z≦3.0. Preferably, the workpiece is a silicon wafer, and the average grain size ratio Z is set to 0.8≦Z≦2.0.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an abrasive grindstone for grinding aworkpiece.

2. Description of the Related Art

An abrasive grindstone containing a boron compound is used to grind aworkpiece formed of a hard brittle material (see Japanese PatentLaid-Open No. 2012-56013, for example). The boron compound has a solidlubricating property and it is therefore considered that the boroncompound functions to suppress the consumption of the abrasivegrindstone due to grinding of the workpiece.

SUMMARY OF THE INVENTION

When a grinding load on the abrasive grindstone is high in grinding aworkpiece formed of any material inclusive of a hard brittle material,the consumption of the abrasive grindstone is also high in general, sothat the frequency of replacement of the abrasive grindstone isincreased. Further, heat generated by grinding is not radiated from theabrasive grindstone, but accumulated therein, so that a grinding speedcannot be increased.

This problem becomes more remarkable in the case of grinding a workpieceformed of a material having low heat conductivity, such as glass.

It is therefore an object of the present invention to provide anabrasive grindstone which can realize a reduction in grinding load, animprovement in heat radiation, or a long life.

In accordance with an aspect of the present invention, there is providedan abrasive grindstone for grinding a workpiece, including diamondabrasive grains and a boron compound; the diamond abrasive grains andthe boron compound being compounded at a predetermined volume ratio; anaverage grain size Y of the diamond abrasive grains being set to 0μm<Y≦50 μm; an average grain size ratio Z of the boron compound to thediamond abrasive grains being set to 0.8≦Z≦3.0.

Preferably, the workpiece is a silicon wafer, and the average grain sizeratio Z is set to 0.8≦Z≦2.0. Preferably, the predetermined volume ratiobetween the diamond abrasive grains and the boron compound is set to 1:1to 1:3. Preferably, the boron compound is selected from group consistingof boron carbide, cubic boron nitride (CBN), and hexagonal boron nitride(HBN).

According to the present invention, it is possible to realize areduction in grinding load on the abrasive grindstone, an improvement inheat radiation, or a long life, so that the productivity can beimproved.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the configuration of a grindingapparatus including an abrasive grindstone according to a preferredembodiment of the present invention;

FIG. 2 is a graph showing the results of grinding of an Si wafer by theabrasive grindstone according to the preferred embodiment;

FIG. 3 is a graph showing the results of grinding of an Si wafer by theabrasive grindstone according to the preferred embodiment;

FIG. 4 is a graph showing the results of grinding of an Si wafer by theabrasive grindstone according to the preferred embodiment; and

FIG. 5 is a graph similar to FIG. 2, showing the results of grinding ofa mirror Si wafer by the abrasive grindstone according to the preferredembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be described indetail with reference to the drawings. The present invention is notlimited to the preferred embodiment. Further, the components used in thepreferred embodiment may include those that can be easily assumed bypersons skilled in the art or substantially the same elements as thoseknown in the art. Further, the configurations described below may besuitably combined. Further, the configurations may be variously omitted,replaced, or changed without departing from the scope of the presentinvention.

FIG. 1 is a perspective view showing the configuration of a grindingapparatus including an abrasive grindstone according to a preferredembodiment of the present invention. In FIG. 1, the X direction shown byan arrow X is the same as the lateral direction of a grinding apparatus10, the Y direction shown by an arrow Y is the same as the longitudinaldirection of the grinding apparatus 10, and the Z direction shown by anarrow Z is the same as the vertical direction perpendicular to the XYplane defined by the X direction and the Y direction.

As shown in FIG. 1, the grinding apparatus 10 includes a first cassette11 for storing a plurality of wafers W as a workpiece before grinding, asecond cassette 12 for storing the wafers W after grinding, handlingmeans 13 serving commonly as means for taking the wafers W out of thefirst cassette 11 before grinding and means for taking the wafers W intothe second cassette 12 after grinding, positioning means 14 forpositioning (centering) the wafers W before grinding, first transfermeans 15 for transferring the wafers W before grinding, second transfermeans 16 for transferring the wafers W after grinding, three chucktables 17, 18, and 19 for holding the wafers W under suction, a turntable 20 adapted to be rotated for rotatably supporting the chuck tables17 to 19, grinding means 30 and 40 as processing means for performingdifferent kinds of grinding to the wafer W held on each of the chucktables 17 to 19, first cleaning means 51 for cleaning the wafers W aftergrinding, and second cleaning means 52 for cleaning the chuck tables 17to 19 after grinding.

In this grinding apparatus 10, one of the wafers W stored in the firstcassette 11 is taken out of the first cassette 11 and then transferredto the positioning means 14 by the handling means 13. Thereafter, thewafer W is positioned by the positioning means 14 and then transferredto one of the chuck tables 17 to 19, e.g., the chuck table 17 in astandby position shown in FIG. 1, by the first transfer means 15. Thethree chuck tables 17 to 19 are arranged at equal intervals in thecircumferential direction of the turn table 20. Each of the chuck tables17 to 19 is rotatable about its axis and movable along a circle on theXY plane by the rotation of the turn table 20. Each of the chuck tables17 to 19 is adapted to be positioned directly below the grinding means(grinding unit) 30 by the counterclockwise rotation of the turn table 20at a predetermined angle, e.g., 120 degrees from the standby position inthe condition where the wafer W is held under suction.

The grinding means 30 functions to perform rough grinding of the wafer Wheld on each of the chuck tables 17 to 19. The grinding means 30 ismounted on a wall portion 22 formed at the rear end of a base 21 in theY direction. A pair of guide rails 31 are provided on the wall portion22 so as to extend in the Z direction, and a support member 33 isslidably mounted on the guide rails 31 so as to be vertically movable bya motor 32. The grinding means 30 is supported to the support member 33,so that the grinding means 30 is vertically movable by the verticalmovement of the support member 33 in the Z direction. The grinding means30 includes a spindle 34 a rotatably supported, a motor 34 for rotatingthe spindle 34 a, a wheel mount 35 fixed to the lower end of the spindle34 a, and a grinding wheel 36 mounted on the lower surface of the wheelmount 35 for grinding the back side of each wafer W. The grinding wheel36 includes a plurality of abrasive grindstones 37 for rough grinding.The abrasive grindstones 37 are fixed to the lower surface of a baseconstituting the grinding wheel 36 so as to be arranged annularly alongthe outer circumference of the base.

The rough grinding is performed in the following manner. When thespindle 34 a is rotated by the motor 34, the grinding wheel 36 isrotated. At this time, the grinding means 30 is lowered in the Zdirection by operating the motor 32 to thereby downward feed thegrinding wheel 36 until the abrasive grindstones 37 come into contactwith the back side of the wafer W held on the chuck table 17, forexample, and positioned directly below the grinding means 30. As aresult, the back side of the wafer W held on the chuck table 17 isground by the abrasive grindstones 37 of the grinding wheel 36 beingrotated. When the rough grinding of the wafer W held on the chuck table17 is ended, the turn table 20 is rotated by 120 degrees in thecounterclockwise direction, so that the wafer W held on the chuck table17 is moved to the position directly below the grinding means (grindingunit) 40. That is, the wafer W is positioned directly below the grindingmeans 40 after rough grinding.

The grinding means 40 functions to perform finish grinding of the waferW held on each of the chuck tables 17 to 19. The grinding means 40 isalso mounted on the wall portion 22. A pair of guide rails 41 areprovided on the wall portion 22 so as to extend in the Z direction, anda support member 43 is slidably mounted on the guide rails 41 so as tobe vertically movable by a motor 42. The grinding means 40 is supportedto the support member 43, so that the grinding means 40 is verticallymovable by the vertical movement of the support member 43 in the Zdirection. The grinding means 40 includes a spindle 44 a rotatablysupported, a motor 44 for rotating the spindle 44 a, a wheel mount 45fixed to the lower end of the spindle 44 a, and a grinding wheel 46mounted on the lower surface of the wheel mount 45 for grinding the backside of each wafer W. The grinding wheel 46 includes a plurality ofabrasive grindstones 47 for finish grinding. The abrasive grindstones 47are fixed to the lower surface of a base constituting the grinding wheel46 so as to be arranged annularly along the outer circumference of thebase. Thusly, the grinding means 40 has the same basic configuration asthat of the grinding means 30, and the abrasive grindstones 47 are onlydifferent in kind from the abrasive grindstones 37.

The finish grinding is performed in the following manner. When thespindle 44 a is rotated by the motor 44, the grinding wheel 46 isrotated. At this time, the grinding means 40 is lowered in the Zdirection by operating the motor 44 to thereby downward feed thegrinding wheel 46 until the abrasive grindstones 47 come into contactwith the back side of the wheel W held on the chuck table 17 andpositioned directly below the grinding means 40. As a result, the backside of the wafer W held on the chuck table 17 is ground by the abrasivegrindstones 47 of the grinding wheel 46 being rotated. When the finishgrinding of the wafer W held on the chuck table 17 is ended, the turntable 20 is rotated by 120 degrees in the counterclockwise direction, sothat the wafer W held on the chuck table 17 is returned to the standbyposition (initial position or load/unload position) shown in FIG. 1. Atthis position, the wafer W whose back side has been finish-ground istransferred to the first cleaning means 51 by the second transfer means16. At the first cleaning means 51, grinding dust is removed from thewafer W by cleaning. Thereafter, the wafer W is taken into the secondcassette 12 by the handling means 13. Further, after the wafer W istransferred from the standby position to the first cleaning means 51,the chuck table 17 in its empty condition is cleaned by the secondcleaning means 52. Although not specifically described above, the roughgrinding and finish grinding of the wafer W held on each of the otherchuck tables 18 and 19 are also similarly performed according to therotational position of the turn table 20. Further, the loading/unloadingof the wafer W to/from each of the other chuck tables 18 and 19 is alsosimilarly performed according to the rotational position of the turntable 20.

Each of the abrasive grindstones 37 and 47 contains diamond abrasivegrains and a boron compound. Examples of the diamond abrasive grainsinclude natural diamond, synthetic diamond, and metal coated syntheticdiamond. Examples of the boron compound include B4C (boron carbide), CBN(cubic boron nitride), and HBN (hexagonal boron nitride). Each of theabrasive grindstones 37 and 47 is obtained by kneading the diamondabrasive grains and the boron compound with a vitrified bond, resinbond, or metal bond, forming the resultant mixture by using a hot press,and then sintering the resultant formed material. Alternatively, each ofthe abrasive grindstones 37 and 47 may be obtained by electroforming thediamond abrasive grains and the boron compound with a nickel plating ona base. Further, the volume ratio between the diamond abrasive grainsand the boron compound is preferably set to 1:1 to 1:3.

Letting X [μm] denote the average grain size of the boron compound and Y[μm] denote the average grain size of the diamond abrasive grains, theaverage grain size ratio Z (=X/Y) of the boron compound to the diamondabrasive grains in each of the abrasive grindstones 37 and 47 is set to0.8 Z 3.0. If the average grain size ratio Z is less than 0.8, thefunction or role of the boron compound as a filler making the abrasivegrindstones 37 and 47 brittle becomes large. Further, if the averagegrain size ratio Z is greater than 3.0, the function or role of thediamond abrasive grains as main abrasive grains becomes smaller than thefunction or role of the filler, so that the diamond abrasive grainshardly contribute to grinding. Further, the average grain size Y of thediamond abrasive grains is set to 0 μm<Y≦50 μm. The reason why theaverage grain size Y of the diamond abrasive grains is set to 50 μm orless is that the use of diamond abrasive grains having an average grainsize of 50 μm or less is suitable for grinding of each wafer W on whichelectronic devices are formed.

In the case that each wafer W as a workpiece in the preferred embodimentis an Si wafer (silicon wafer) containing Si, the average grain size Yof the diamond abrasive grains in each abrasive grindstone 37 for roughgrinding is preferably set to 20 μm≦Y≦50 μm because the average grainsize for rough grinding is larger than that for finish grinding.Further, the average grain size Y of the diamond abrasive grains in eachabrasive grindstone 47 for finish grinding is preferably set to 0.5μm≦Y≦1 μm because the average grain size for finish grinding is smallerthan that for rough grinding.

By setting the average grain size ratio Z of the boron compound to thediamond abrasive grains to 0.8≦Z≦3.0 and setting the average grain sizeY of the diamond abrasive grains to 0 μm<Y≦50 μm as described above, thesolid lubricating property of the boron compound can be effectivelydeveloped in grinding each wafer W. Furthermore, the grinding load onthe abrasive grindstones 37 and 47 can be reduced. Since the grindingload on the abrasive grindstones 37 and 47 is reduced, the consumptionof the abrasive grindstones 37 and 47 in grinding each wafer W with theabrasive grindstones 37 and 47 can be reduced to result in a long life.Further, the boron compound has high heat conductivity. In particular,CBN and HBN have high heat conductivity. Accordingly, heat radiationfrom a working point can be improved in grinding the workpiece with theabrasive grindstones 37 and 47. Thusly, the degree of consumption of theabrasive grindstones 37 and 47 in the grinding apparatus 10 can besuppressed to thereby reduce the frequency of replacement of theabrasive grindstones 37 and 47. As a result, the productivity in thegrinding apparatus 10 can be improved.

A comparison was made between a conventional abrasive grindstone and theabrasive grindstone according to the present invention. FIGS. 2 to 4 aregraphs showing the results of grinding by the abrasive grindstoneaccording to the preferred embodiment. In FIGS. 2 and 4, the verticalaxis represents amperage [A] of an electric current supplied to themotor for rotating the abrasive grindstone, and the horizontal axisrepresents grinding time [sec] required for grinding of each wafer W. InFIG. 3, the vertical axis represents consumption [μm], and thehorizontal axis represents the number of wafers W ground, wherein eachdot represents the consumption of the abrasive grindstone at the end ofgrinding of each wafer W.

The conventional abrasive grindstone (which will be hereinafter referredto as “conventional sample”) and the abrasive grindstone according tothe present invention (which will be hereinafter referred to as“invention samples 1 to 4”) are both abrasive grindstones for roughgrinding. That is, the “invention samples 1 to 4” are examples of eachabrasive grindstone 37. On the other hand, the “conventional sample” isan abrasive grindstone excluding a boron compound and containing onlydiamond abrasive grains, wherein the average grain size Y of the diamondabrasive grains is 20 μm. Each of the “invention samples 1 to 4” is anabrasive grindstone containing both diamond abrasive grains and CBN as aboron compound, wherein the diamond abrasive grains and the boroncompound are kneaded together with a vitrified bond and then sintered.The “invention sample 1” is defined so that the average grain size X ofthe boron compound is 20 μm, the average grain size Y of the diamondabrasive grains is 20 μm, the average grain size ratio Z is 1, and thevolume ratio between the boron compound and the diamond abrasive grainsis 1. The “invention sample 2” is defined so that the average grain sizeX of the boron compound is 30 μm, the average grain size Y of thediamond abrasive grains is 20 μm, the average grain size ratio Z is 1.5,and the volume ratio between the boron compound and the diamond abrasivegrains is 1. The “invention sample 3” is defined so that the averagegrain size X of the boron compound is 45 μm, the average grain size Y ofthe diamond abrasive grains is 20 μm, the average grain size ratio Z is2.25, and the volume ratio between the boron compound and the diamondabrasive grains is 1. The “invention sample 4” is defined so that theaverage grain size X of the boron compound is 50 μm, the average grainsize Y of the diamond abrasive grains is 20 μm, the average grain sizeratio Z is 2.5, and the volume ratio between the boron compound and thediamond abrasive grains is 1. Each wafer W as a workpiece to be groundby the “conventional sample” and the “invention samples 1 to 4” is an Siwafer having an oxide film (SiO₂ film having a thickness of about 600nm) present on the work surface. That is, a plurality of such wafers Wwere ground by the “conventional sample” and the “invention samples 1 to4.”

FIG. 2 shows the results of grinding of the wafers W by the“conventional sample” and the “invention sample 1.” In FIG. 2, QS1 andPS1 denote the results of grinding of the first wafer W by the“conventional sample” and the “invention sample 1,” respectively; QS2and PS2 denote the results of grinding of the second wafer W by the“conventional sample” and the “invention sample 1,” respectively; andQS3 and PS3 denote the results of grinding of the third wafer W by the“conventional sample” and the “invention sample 1,” respectively. Asapparent from FIG. 2, the results of grinding by the “conventionalsample” (QS1 to QS3) are such that no remarkable peaks are present atthe start of grinding and the amperage is uniform over the grinding timeregardless of the number of wafers W to be ground. On the other hand,the results of grinding by the “invention sample 1” (PS1 to PS3) aresuch that peaks higher than the amperage in the case of the“conventional sample” appear at the start of grinding, but the amperageafter the occurrence of the peaks is remarkably lower than the amperagein the case of the “conventional sample” regardless of the number ofwafers W to be ground. In the case of the “invention sample 1” (alsosimilarly in the case of the “invention samples 2 to 4”), the nativeoxide (SiO₂) formed on the work surface of each wafer W is ground at thestart of grinding, so that the amperage showing a grinding load at thestart of grinding is higher than that in the case of the “conventionalsample” and appears as the peaks. However, after removing the nativeoxide by grinding, the amperage is rapidly decreased. In other words,the grinding load is greatly reduced as a whole.

As described above, the amperage in grinding each wafer W by the“invention sample 1” is lower than that by the “conventional sample” asshown in FIG. 2. Accordingly, the consumption of the “invention sample1” in grinding each wafer W is remarkably reduced as shown in FIG. 3. Asa result, in the case of grinding the plural wafers W, the gradient ofthe consumption of the “invention sample 1” (the slope of a line PSshown in FIG. 3) is remarkably smaller than the gradient of theconsumption of the “conventional sample” (the slope of a line QS shownin FIG. 3). That is, since the grinding load on the “invention sample 1”is lower than that on the “conventional sample,” the consumption of the“invention sample 1” is lower than that of the “conventional sample,” sothat the life of the “invention sample 1” is longer than that of the“conventional sample.”

FIG. 4 shows the results of grinding of the n-th wafer W (n is apredetermined number) by the “conventional sample” and the “inventionsamples 1 to 4.” In FIG. 4, QS4 denotes the result of grinding of then-th wafer W by the “conventional sample”; PS4 denotes the result ofgrinding of the n-th wafer W by the “invention sample 1”; PS5 denotesthe result of grinding of the n-th wafer W by the “invention sample 2”;PS6 denotes the result of grinding of the n-th wafer W by the “inventionsample 3”; and PS7 denotes the result of grinding of the n-th wafer W bythe “invention sample 4.” As apparent from FIG. 4, the result ofgrinding by the “conventional sample” (QS4) is such that no remarkablepeak is present at the start of grinding of the oxide film and theamperage is uniform (15 to 16 amperes) over the grinding time. On theother hand, the result of grinding by the “invention sample 1” (PS4) issuch that a peak (about 15 amperes) less than the amperage in the caseof the “conventional sample” appears at the start of grinding and theamperage after the occurrence of the peak is remarkably lower (12 to 13amperes) than the amperage in the case of the “conventional sample.”

The result of grinding by the “invention sample 2” (PS5) is such that apeak (about 16 amperes) higher than the amperage in the case of the“conventional sample” and the amperage in the case of the “inventionsample 1” appears at the start of grinding and the amperage after theoccurrence of the peak is remarkably lower (about 12 amperes) than theamperage in the case of the “conventional sample” and substantially thesame as the amperage in the case of the “invention sample 1.” The resultof grinding by the “invention sample 3” (PS6) is such that a peak (about18 amperes) higher than the amperage in the case of the “conventionalsample” and the amperage in the case of the “invention samples 1 and 2”appears at the start of grinding and the amperage after the occurrenceof the peak is remarkably lower (about 12 amperes) than the amperage inthe case of the “conventional sample” and substantially the same as theamperage in the case of the “invention samples 1 and 2.” The result ofgrinding by the “invention sample 4” (PS7) is such that a peak (about 18amperes) higher than the amperage in the case of the “conventionalsample” and the amperage in the case of the “invention samples 1 to 3”appears at the start of grinding and the amperage after the occurrenceof the peak is remarkably lower than the amperage in the case of the“conventional sample” and substantially the same as the amperage in thecase of the “invention samples 1 to 3.” In the case of the “inventionsamples 1 to 4,” the amperage increases from 9 amperes to the peak atthe start of grinding (in grinding the oxide film) and thereafterdecreases to 12 to 13 amperes in grinding the Si wafer. This resultshows that the grinding load after removing the oxide film is greatlylower than that in the case of the “conventional sample.”

In the case of the “invention samples 1 to 4,” the amperage in grindingeach wafer W is lower than that in the case of the “conventionalsample.” Accordingly, the grinding load in grinding the plural wafers Wis lower than that in the case of the “conventional sample,” so that theconsumption of the “invention samples 1 to 4” is lower than that in thecase of the “conventional sample.” As a result, the life of the“invention samples 1 to 4” is longer than that of the “conventionalsample.” In particular, the “invention samples 1 and 2” are moresuitable than the “invention samples 3 and 4” in grinding an Si wafer asthe wafer W as the workpiece because the peak at the start of grindingis lower. Accordingly, in the case that the workpiece is an Si wafer,the average grain size ratio Z is preferably set to 0.8≦Z≦2.0. Further,the amperage shown in FIG. 4 changes according to the grinding apparatusand the peak is preferably lower from the viewpoints of low grindingload and application to the grinding apparatus. That is, the “inventionsample 1” or the “invention sample 2” is more preferable than the“invention sample 3” or the “invention sample 4.”

While an Si wafer having an oxide film (native oxide) formed on the worksurface is used as the workpiece in the preferred embodiment, the waferW as the workpiece is not limited to an Si wafer, but the wafer W may bean SiC wafer containing SiC, for example. In this case, the averagegrain size Y of the diamond abrasive grains in each abrasive grindstone37 for rough grinding is preferably set to 3 μm≦Y≦10 μm because theaverage grain size for rough grinding is larger than that for finishgrinding. Further, the average grain size Y of the diamond abrasivegrains in each abrasive grindstone 47 for finish grinding is preferablyset to 0.5 μm≦Y≦1 μm because the average grain size for finish grindingis smaller than that for rough grinding. Further, the average grain sizeratio Z is preferably set to 1.0≦Z≦2.0.

Further, the wafer W as the workpiece may be a mirror Si wafer. FIG. 5is a graph showing the results of grinding by the abrasive grindstoneaccording to the present invention in the case that the wafer W is amirror Si wafer. In FIG. 5, the vertical axis represents amperage [A] ofan electric current supplied to the motor for rotating the abrasivegrindstone, and the horizontal axis represents grinding time [sec]required for grinding of each wafer W. FIG. 5 shows the results ofgrinding of a mirror Si wafer by the “conventional sample” and the“invention sample 1.” The mirror Si wafer is an Si wafer having a mirrorsurface, wherein no oxide film is formed on the mirror surface or a thinoxide film is formed on the mirror surface with the thickness smallerthan the thickness of the oxide film formed on the Si wafer shown inFIGS. 2 to 4. In FIG. 5, QM1 and PM1 denote the results of grinding ofthe first wafer W by the “conventional sample” and the “invention sample1,” respectively, and QM2 and PM2 denote the results of grinding of thesecond wafer W by the “conventional sample” and the “invention sample1,” respectively. As apparent from FIG. 5, the results of grinding bythe “conventional sample” (QM1 and QM2) are such that no remarkablepeaks are present at the start of grinding and the amperage is uniform(about 18 amperes at the maximum) over the grinding time regardless ofthe number of wafers W to be ground. On the other hand, the results ofgrinding by the “invention sample 1” (PM1 and PM2) are such that nopeaks appears at the start of grinding and the amperage is lower (about16 amperes at the maximum) than that in the case of the “conventionalsample.” The mirror Si wafer also contains Si as a main component and itis considered that the grinding behavior in grinding the mirror Si waferis similar to that in grinding the Si wafer shown in FIGS. 2 to 4 afterremoving the silicon oxide film. Accordingly, also in grinding themirror Si wafer by using the “invention samples 2 to 4,” it is possibleto exhibit effects similar to those in the case of the Si wafer shown inFIGS. 2 to 4.

As described above, the amperage in grinding each wafer W by the“invention sample 1” is lower than that by the “conventional sample.”Accordingly, also in the case of grinding the mirror Si wafer, thegrinding load on the “invention sample 1” is lower than that on the“conventional sample.” As a result, the consumption of the “inventionsample 1” in grinding each wafer W is lower than that of the“conventional sample,” so that the life of the “invention sample 1” islonger than that of the “conventional sample.” Further, since the boroncompound has high heat conductivity, heat radiation from the grindingpoint on the workpiece to be ground by the abrasive grindstones 37 and47 can be improved.

Accordingly, also in the case of grinding the mirror Si wafer, it ispossible to suppress a reduction in grinding speed in consideration ofheat generated in grinding, so that the productivity can be improved.

The present invention is not limited to the details of the abovedescribed preferred embodiment. The scope of the invention is defined bythe appended claims and all changes and modifications as fall within theequivalence of the scope of the claims are therefore to be embraced bythe invention.

What is claimed is:
 1. An abrasive grindstone for grinding a workpiece,comprising diamond abrasive grains and a boron compound; said diamondabrasive grains and said boron compound being compounded at apredetermined volume ratio; an average grain size Y of said diamondabrasive grains being set to 0 μm<Y≦50 μm; an average grain size ratio Zof said boron compound to said diamond abrasive grains being set to0.8≦Z≦3.0.
 2. The abrasive grindstone according to claim 1, wherein saidworkpiece is a silicon wafer, and said average grain size ratio Z is setto 0.8≦Z≦2.0.
 3. The abrasive grindstone according to claim 1, whereinsaid predetermined volume ratio between said diamond abrasive grains andsaid boron compound is set to 1:1 to 1:3.
 4. The abrasive grindstoneaccording to claim 1, wherein said boron compound is selected from thegroup consisting of boron carbide, cubic boron nitride (CBN), andhexagonal boron nitride (HBN).